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Tapping into the Instrument Panel Dimmer

Hello all,
I am new to the forum and want to say Hello to everyone. I have been watching for a while and have seen some great threads.

I have a 05 Forester and I just bought and installed a compass (not the LCD in the mirror kind, but the dial floating in a liquid kind.) I have mounted it on the top of the dash in the center. It came with wiring attached to power a small LED light for night use which I have not hooked up yet. I can see where I could tap into the fuse panel and through a switch to make the connection, but wonder if it might be feasible to tap into the dash panel lighting, so that it goes on with the rest of the dash lights.

It sounds like you're on the right track. I would wonder if you couldn't get your wires to follow the top of the dash pad at the glass, and press it out of view into the crack there all the way to the A pillar, then down under the dash, along to the wiring harness for the heater controls. Here you could tap into the sources needed without having to stand on your head under the dash trying to find another source. You don't give a year and model so I can't be more specific. Taking the garnishes around the heater controls is not a big deal, if you take your time and don't force anything. Good luck!

Keep in mind that an LED doesn't work the same as a light bulb. It may not come on until the dimmer is set to a certain point and the diode junction is "energized", and I don't believe you'll see much dimming action once it is on.

If you want to use the dimmer, it's my opinion that you'll have to replace the LED circuit with a 12 volt peanut bulb of some type from Radio Shack.

Does the ligher receptacle have a light ring around it that dims? That may be the easiest place to tap into. There should only be a ground, 12 volts for the lighter, and the lighting wire.

I have a related thread that I have often wondered about, but never really gotten into. I have a 96 Legacy Outback and the temp gauge doesn't work. Is it just as simple as removing the screws to get at the back of the instrument panel, or does it become a real can of worms and something I should not attempt.

In my 96 Leg, I loosened the two bolts that hold the steering column in place until they were just about all the way out. The gives enough clearance to wrestle the cluster out of there after you remove the screws.

Keep in mind that an LED doesn't work the same as a light bulb. It may not come on until the dimmer is set to a certain point and the diode junction is "energized", and I don't believe you'll see much dimming action once it is on.

Correct.

One could also use a pulse width modulation (PWM) IC to take the analog voltage from the dimmer circuit and convert it to a variable duty cycle square wave to modulate the brightness of the LED. A programmable microcontroller would be the easiest way to do this.

Another way to do it would be to use an operation amplifier and use the dimmer voltage to modulate the LED current. This would be more difficult but much more elegant.

Some car makers (GM for certain) are now using PWM for dimming, so I wouldn't rule that out. You might just try hooking it up first, because if Subaru is now using PWM dimming it will work perfectly.